Change fast_forward_to() to check if locking the ref failed, print a
nice error message and bail out early.

The old code did not check if ref_lock was NULL and relied on the
fact that the write_ref_sha1() would safely detect this condition
and set the return variable ret to indicate an error.

While that is safe, it makes the code harder to read for two reasons:

 * Inconsistency.  Almost all other places we do check the lock for
   NULL explicitly, so the naive reader is confused "why don't we
   check here?"

 * And relying on write_ref_sha1() to detect and return an error for
   when a previous lock_any_ref_for_update() failed feels obfuscated.

This change should not change any functionality or logic aside from
adding an extra error message when this condition is triggered
(write_ref_sha1() returns an error silently for this condition).
Signed-off-by: NRonnie Sahlberg <sahlberg@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>